This invention relates to improvements in an ion microprobe analyzer. More particularly, it relates to an ion microprobe analyzer capable of analyses with high precision.
As is well known, the ion microprobe analyzer (hereinafter abbreviated to IMA) is an analytical apparatus for solids and is excellent for microanalysis, in-depth analysis, and surface analysis. An example of a prior art IMA is illustrated in FIG. 1. When broadly separated in function, the IMA is constructed of a primary illumination system, a secondary ion spectrometer and a scanning ion microscope.
The primary illumination system is composed of an ion source 1, condenser lenses 2, object lenses 3, a primary ion scanning deflector 4, an objective aperture 5, a specimen 6, and a shield electrode 7. The function of the primary illumination system is to contract an ion beam 8, generated from the ion source 1, by the two stages of lenses 2 and 3 and to project the ion beam onto the specimen 6. The deflector 4 has the function of scanning the fine contracted beam 8 on the specimen as in a television set. The ion source 1, the two stages of lenses 2 and 3, the objective aperture 5, and the deflector 4 in the primary illumination system are all arranged on one axis.
The secondary ion spectrometer is composed of a secondary ion extractor 9, lenses 10 for correcting the path of a secondary ion beam, an electrostatic sector 11, a .beta.-slit 12, a magnetic sector 13, a C-slit 14, a secondary detector 15, and a recorder 16 or a counter (not shown). It functions as stated below.
Secondary ions based on specimen atoms as produced from the specimen irradiated by the primary ions are extracted by an electric field which is established by the secondary ion extractor 9. They are led to the electrostatic sector 11, and are subjected to energy separation by the .beta.-slit 12. Subsequently, the secondary ions having specific energy which have passed through the .beta.-slit 12 are introduced into the magnetic sector 13 and are sorted on the basis of the mass to charge ratio (M/e), and are detected as specific ion currents by the C-slit 14. The mass spectrum is obtained by gradually varying the field intensity of the magnetic sector 13 with the other conditions fixed. In order to detect the intensity of the secondary ions, a secondary electron multiplier is chiefly utilized.
The scanning ion microscope is composed of a cathode ray tube (hereinbelow, abbreviated to CRT) 17, the operating principle of which with respect to the remaining system is as stated below. First, the finely contracted primary ion beam 8 and the electron beam of the CRT 17 are synchronously scanned by the scanning power supply 31. Subsequently, the image of a specific element or the image of the unevenness of the surface of the specimen is obtained on the CRT 17 by utilizing as the video signal of the CRT 17 a signal obtained by the secondary ion detector 15.
The prior art IMA has inherent problems which will now be discussed, and particularly two contradictory points which are the more significant problems:
i. High-speed neutral particles generated in the ion source 1 get onto the specimen and produce secondary ions which result in the generation of noise. When, for example, a duoplasmatron type ion source is employed as the ion source 1, some ions formed within the ion source are neutralized before getting out of the ion source, and hence, high-speed neutral particles are generated. The high-speed neutral particles have a speed substantially equal to that of the ion beam. In the case where the air pressure of the ion source is 10.sup.-1 Torr, the neutral particles generated amount to about 25% of the ion beam.
ii. The specimen is irradiated by ions which are charged particles. Therefore, in the case where the specimen is an insulator, or where its surface is covered with a nonconductive thin film, the surface of the specimen accumulates an electrical charge and a good analysis thereof becomes difficult.
The problem concerning the neutral particles will be explained in detail in conjunction with FIGS. 2a and 2b, which illustrate the influence of the neutral particles on the operation of the apparatus. Referring to FIG. 2a, the ion beam 8 can have its diameter on the specimen arbitrarily varied by the lens systems 2 and 3. In contrast, the neutral particles indicated by dotted lines in the figure have no charge and are not subject to the focusing action of the lens systems, and accordingly, they advance from the ion source 1 and get to the specimen through the objective aperture 5. The size of the neutral beam 19 on the specimen changes depending on the size of the objective aperture 5, which is in general, several mm.
Now consider a case where the specimen to be analyzed consists of a nickel mesh, as shown in FIG. 2b. In this case, the secondary ions which enter the mass spectrometer after passing through the extractor 9 are of two kinds: the secondary ions resulting from the excitation of the specimen by the primary ion beam, and those owing to the excitation of the specimen by the neutral particles. Moreover, regarding the latter group of ions, the beam diameter is determined by the size of the objective aperture 5 and is large as compared with the diameter of the primary ion beam. Consequently, even when the primary ion beam 8 is projected between the meshes as shown in FIG. 2b, the secondary ions which are generated from a neutral particle irradiation region 20, indicated by a circle in the figure, come into incidence on the spectrometer, and they are detected as noise.
FIGS. 3a and 3b show examples of analyses by the prior art method. FIG. 3a corresponds to a point of analysis A indicated in FIG. 2b, and illustrates the mass spectrum of nickel. FIG. 3b illustrates a spectrum in the case where the primary ion beam was caused to fall on a point B (in FIG. 2b) between the meshes, i.e., a part at which no nickel existed. Although properly the spectrum of nickel ought not to be detected, the nickel spectrum having an intensity of about 1/4 of the value on the nickel mesh was obtained, as shown in these figures. It is evident that such nickel spectrum is attributed to the high-speed neutral particles. In this manner, in the analysis of a very small part of the specimen by the IMA, unless the neutral particles are removed, the analytical precision of the apparatus is lowered conspicuously and a high-precision analysis becomes difficult. Further, although no explanation is made here, the lowering of the analytical precision is also apparent in the in-depth analysis.
The problem concerning surface charge formation will now be explained. In the IMA, an ion beam is employed as an exciting source. Therefore, where the specimen is an insulator or its surface is covered with a nonconductive film, the charge accumulation pnenomenon occurs. Besides, when charged particles are exploited in the case of executing the analysis of a function device, such as an integrated circuit (IC), the electrical characteristics of the IC change under the influence thereof and the reuse of the IC becomes difficult though the cause is not clear.
Regarding this problem, there has been utilized a method wherein charges on the specimen surface are neutralized by superposingly irradiating the ion beam irradiation portion with a low-speed electron beam. Also, a method has been used wherein the charge accumulation is avoided by employing negative ions as the primary ions. In this case, the secondary electrons serve to neutralize the charges. With either method, however, the charged particles are exploited for the primary beam, and the settlement of the latter problem on the electrical characteristics cannot be expected.